A linear actuator consisting of a screw mechanism for converting a rotational movement of an electric motor into a linear movement of an output member is widely used as actuators in power seats for vehicles which allow various parts of the seat to be adjusted. In such a linear actuator, a nut member provided with an internal thread is supported by a frame so as to be rotatable by an electric motor but to be immovable in the axial direction, and a screw rod provided with an external thread is threadably engaged with the nut member so as to be axially moveable but not to be rotatable. Thus, by rotating the nut member around its axial center line, the nut member is made to undergo a linear movement.
In such a screw mechanism, it is necessary to provide a certain amount of play or a dimensional tolerance between the threads of the screw rod and the nut member so that the smooth operation of the screw mechanism may be ensured without requiring any undue dimensional accuracy in the fabrication process.
Because a vehicle seat is subjected to acceleration and deceleration, it is necessary to be sufficiently rigid for the vehicle occupant to be comfortably seated. If there is any excessive play in the threading engagement, it will not only cause discomfort to the vehicle occupant but also undesirable impulsive stresses to various parts of the screw mechanism which are detrimental to the durability of the power seat mechanism. On the other hand, if an excessive tight fit is used for the threading engagement, the friction in the screw mechanism may become unacceptably great. If the dimensional tolerance of the threading engagement is made more rigorous so that proper fit may be ensured at all times, some difficulty may be created in controlling the dimensional precision, and the fabrication cost will be increased.